Combustion engines rely on a stable fuel supply to initiate and maintain a desired combustion rate. For example, combustors on a gas turbine ignite fuel to generate combustion gases having a high temperature, pressure, and velocity. A fuel injection system supplies fuel to the combustors for ignition by a flame. At low power levels, the fuel injection system must provide fuel with a relatively high calorie content to maintain the combustion and avoid “blow out” of the flame. Conversely, “blow out” is less of a concern at high power levels, and fuel with a lower calorie content is more economical.
Possible fuels used by commercial combustion engines include blast furnace gas, coke oven gas, natural gas, and propane. The calorie content varies between each of these fuels. In addition, the calorie content for any particular fuel may vary, depending on the source of the fuel and the physical characteristics of the particular fuel, such as the purity, temperature, and pressure. For example, blast furnace gas and coke oven gas are by-products from the combustion of coke in the iron and steel industry; whereas, natural gas and propane are processed from naturally occurring underground deposits of methane and petroleum. The calorie content of blast furnace gas, also known as converter or LD gas, can vary between 700 kCal/m3 and 950 kCal/m3. The calorie content of coke oven gas can vary between 3900 kCal/m3 and 4400 kCal/m3. The calorie content of natural gas and petroleum often exceeds 4100 kCal/m3.
The unit cost of fuel generally increases as the calorie content of the fuel increases. Therefore, various systems and methods exist to reduce fuel costs by mixing less expensive, lower calorie content fuel with more expensive, higher calorie fuel to obtain a mixed fuel having a desired calorie content.
For example, U.S. Pat. No. 7,396,228 describes a fuel gas calorie control method and device that mixes multiple fuels having different calorie contents to obtain a mixed fuel having a desired calorie content. The system and method relies on the measured flow rate and measured calorie content of the constituent fuels to calculate and predict the resulting calorie content of the mixed fuel.
Various factors can effect the accuracy of the calculations used to predict the resulting calorie content of the mixed fuel. For example, an accurate flow measurement depends on the pressure of the supplied fuels, and the pressure of the supplied fuels may change over time. In addition, the constituent fuels are often supplied through large volume, low pressure piping, which further effects the accuracy of any flow measurement.
System processes may further change either the actual or desired calorie content of the mixed fuel. For example, the mixed fuel may be pressurized before introduction into the combustion engine, changing the calorie content of the mixed fuel. Moreover, the optimum or desired calorie content of the mixed fuel may change based on changes in the operating level of the combustion engine.
Therefore, the need exists for a fuel control system that does not rely on an accurate measurement of the constituent fuel flow to produce a mixed fuel having a desired calorie content. In addition, the need exists for a fuel control system that can adjust the calorie content of the mixed fuel to account for changes in the calorie content caused by subsequent processing of the mixed fuel after mixing. Lastly, the need exists for a fuel control system that can adjust the desired calorie content of the processed mixed fuel supplied to the combustion engine based on changes in the operating level of the combustion engine.